Field of the Invention (Technical Field)
The present invention relates to steering of phased-array antenna systems and to construction of optical circuitry.
Background Art
Phased array antennas exhibit desirable properties for communication and radar systems. One primary feature of these antennas is that they require no mechanical motion for beam steering. This allows for very rapid beam steering and excellent reliability. Another primary feature is the ability to bring high power to the target while minimizing typical microwave power combining losses. Unfortunately, phased array antennas are also very complex electronic systems. They require relatively complex and heavy transmit and receive electronics at the antenna location and/or significant microwave distribution systems. Large arrays and millimeter wave frequencies contribute to these disadvantages. These issues have effectively limited the use of phased-array antennas.
Phased-array antennas offer at least three principal benefits over conventional mechanically steered antennas:
1. Electronically controlled phased-array antennas are "inertia-less." They can be "instantaneously" pointed in any direction. No mechanical movement is required, which also means no mechanical wear will occur.
2. The antenna can radiate and receive in multiple directions simultaneously.
3. Phased-array antennas can be built with MMIC chips at each antenna element (forming the so-called "active array" antenna) allowing for very large effective-radiated power levels and large system redundancy.
Electronically-controlled phased-array antennas are not only useful for radar systems. They are useful for any application requiring a steerable antenna (e.g., communication systems).
Phased-array antennas produced today are extremely expensive. One of the reasons for their large expense is the great complexity of electrical interconnects for radio frequency (RF) signal distribution and phase control. The present invention addresses this deficiency by replacing virtually all of the interconnects with a single photonic integrated circuit and inexpensive fiber-optic cables.
Control of arbitrarily large phased-array antennas from a lightweight, compact unit represents a significant challenge in the implementation of phased-array radar for airborne systems. Current systems use large amounts of bulky copper cable with RF splitters and phase shifters to distribute RF power to the antenna and control the phase of each element. This approach often results in desk-sized control units weighing many hundreds of pounds. Electronically steerable phased-array antennas are being used today almost exclusively in military systems. This is largely true due to their great expense.
The present invention recasts the same system configuration in lightweight optical fiber with optical phase-shift elements and interconnecting optical circuitry made from GaAs/AlGaAs or lithium niobate waveguides. This approach will result in a significantly lighter and less costly system. The present invention provides an optical system which greatly reduces the phase control electronics and electrical interconnect complexity normally associated with a phased-array antenna. The invention permits reduction of the system cost of such antenna, and thus allows for many commercial applications to become economically feasible.
Photonic integrated circuits (PICs) have not evolved to the level of sophistication of electronic integrated circuits. One primary impediment to the development of a wide variety of PICs for many applications is the difficult design and fabrication process required of every new PIC. Development of a fully functional PIC requires careful consideration of the application requirements, the detailed behavior of each device used, and the interaction of all the devices with the others. Generally, custom processing is required in order to build the circuit, requiring the engineer designing the circuit to have a full understanding of the process capabilities and limitations. This is in sharp contrast to the electronic integrated circuit field in which well established device and circuit design rules based on proven process technology allow engineers to easily lay out integrated circuits. The engineer requires only a minimum understanding of the actual operation of the devices in the circuit and the process technology required to manufacture the circuit. This ease of design and the ability of the designer to view the manufacturing process as a black box from which useful components emerge (provided adherence to the design rules) has allowed a wide diversity of electronic integrated circuits to flourish in the marketplace.
In order for PICs to become widely accepted as viable solutions in a variety of applications, the huge overhead cost associated with custom design and manufacture of each circuit must be minimized. One way to minimize costs is to follow the example of the electronic integrated circuit industry by establishing a fixed set of mutually compatible devices with well-established design rules governing their placement in the circuit in such a manner as to assure an operating PIC. The present invention includes one such optical device set and covers several application circuits which may be realized with the device set.